Add a native workspace directory picker to onboarding

Onboarding only accepted a typed workspace path, which is error-prone and
diverged from the original wizard. Add a pickDirectory IPC backed by Electron's
open-directory dialog and a Choose button beside the path field, so the founder
can select a real folder for agent artifacts.

AgentCompany-New/src/shared/desktop-api.ts

@@ -97,6 +97,7 @@ export interface DesktopApi {
   createScheduledPost(input: CreateScheduledPostInput): Promise<DesktopResult<AppSnapshot>>;
   setScheduledPostState(input: SetScheduledPostStateInput): Promise<DesktopResult<AppSnapshot>>;
   importFile(input: ImportFileInput): Promise<DesktopResult<AppSnapshot>>;
+  pickDirectory(): Promise<DesktopResult<string | null>>;
   setFilePinned(input: SetFilePinnedInput): Promise<DesktopResult<AppSnapshot>>;
   createMigrationReport(input: CreateMigrationReportInput): Promise<DesktopResult<AppSnapshot>>;
   createRollbackSnapshot(input: CreateRollbackSnapshotInput): Promise<DesktopResult<AppSnapshot>>;

AgentCompany-New/src/shared/ipc.ts

@@ -35,6 +35,7 @@ export const IPC_CHANNELS = {
   createScheduledPost: "agentcompany:create-scheduled-post",
   setScheduledPostState: "agentcompany:set-scheduled-post-state",
   importFile: "agentcompany:import-file",
+  pickDirectory: "agentcompany:pick-directory",
   setFilePinned: "agentcompany:set-file-pinned",
   createMigrationReport: "agentcompany:create-migration-report",
   createRollbackSnapshot: "agentcompany:create-rollback-snapshot",